Objective  Various plant growth regulators (PGRs) capable of reducing plant height and improve lodging resistance without affecting crop yield were tested on the spring foxtail millet. Optimal spraying time of the selected agents were determined based on the effects on growth, development, and yield of the plants after the treatments.

  Method  On the spring foxtail millet, Changsheng 07, at the seedling or jointing stage, 5 PGRs for the treatment, as well as water as control, were sprayed separately. At booting and maturity stages, chlorophyll, photosynthetic rate, dry matters accumulation, major agronomic traits, and yield of the plants were measured for comparison.

  Result  Among PGRs applied, chlormequat, paclobutrazol, and mepiquat chloride were found to dwarf the plants in varying degrees. The treatments increased the stem diameter, altered the secondary root number and weight, improved the lodging resistance, and increased the chlorophyll content and photosynthetic rate of the plants over control. Even the crop yield rose under the treatments as the plant tissues continued to propagate and accumulate at the booting stage. Spraying of the PGRs at seedling stage produced greater effects than at jointing stage, and the chlormequat application was more effective than the other PGRs. Compared to control, the treated plants were significantly shorter on height (decreased by 8.16%), larger in stem diameter (increased by 6.67%), more secondary roots with count increased (rose by 18.18%), greater net photosynthetic efficiency (higher by 16.07%) (P<0.05), and higher yield with 5.58% increase in 2016 and 5.67% in 2017. Less effective than chlormequat, paclobutrazol sprayed at seedling stage caused a plant height reduction of 4.19%, a stem girth enlargement of 5.00%, and a root weight increase of 34.3%, while at the jointing stages, those of 2.24%, 3.21%, and 30.4%, respectively, in comparison to those of control. Among the three agents, mepiquat chloride was least effective in dwarfing the plant. After spraying at the seedling and jointing stages, the plant height decreased merely by 2.31% and 0.79%, respectively, while the stem thickness increased by 4.1% and 1.67%, respectively, over control. The application of ethephon at jointing stage also significantly dwarfed the plant with a 5.38% increase on stem thickness, 21.21% increase on secondary root count, 39.6% increase on root weight, 8.54% increases on dry matters and aboveground plant weights, but significant decreases on chlorophyll content and net photosynthetic rate, over control.

  Conclusion  The plant height and stem girth of the existing mid-and-late-maturing spring foxtail millet cultivars could be altered by spraying chlormequat or paclobutrazol at seedling stage or ethephon at jointing stage to facilitate mechanical cultivation, increase lodging resistance, and improve yield of the hay stock.